docs: def subsample(self, rate):
          """Returns a subsampled version of the NormalCloud.
  
          Parameters
          ----------
          rate : int
              Only every rate-th element of the NormalCloud is returned.
  
          Returns
          -------
          :obj:`RateCloud`
              A subsampled point cloud with N / rate total samples.
  
          Raises
          ------
          ValueError
              If rate is not a positive integer.
          """
          if type(rate) != int and rate < 1:
              raise ValueError('Can only subsample with strictly positive integer rate')
          subsample_inds = np.arange(self.num_points)[::rate]
          subsampled_data = self._data[:,subsample_inds]
          return NormalCloud(subsampled_data, self._frame)